The invention relates in general to tube-launched projectiles and in particular to tube-launched projectiles that are statically stabilized.
Tube-launched projectiles may require adequate aerodynamic stability to damp out launch disturbances and maintain a low angle of attack throughout flight. Projectiles may be stabilized one of two ways, statically or gyroscopically.
A projectile may be considered statically stable if its aerodynamic center of pressure is aft of its center of gravity (CG). The distance between the aerodynamic center of pressure and the CG is known as the static stability margin, or SSM. As a rule of thumb, a projectile may be considered to possess adequate static stability if the SSM is at least 10% of the overall body length.
When designing a projectile, stabilizing surfaces (such as tail fins, flares) may be placed near the aft end of the projectile to produce an aerodynamic restoring moment. The stabilizing surfaces may be designed to produce enough lift so that the projectile has an adequate SSM. If the projectile is subjected to launch disturbances that cause it to rotate to an angle of attack, the restoring moment may realign the projectile axis with the velocity vector. Aerodynamic theory and empirical data may help select methods for producing the largest possible stabilizing moment at the desired flight conditions, while not causing undesirable side effects, such as high aerodynamic drag. The greatest benefits may be obtained by employing large lifting surfaces as far behind the CG as possible.
Most projectiles have practical limitations on the size (length or diameter) of tail stabilizers. Tube-launched projectiles, in particular, may be constrained in that no part of the body may be larger than the inside diameter of the tube. Some projectiles bodies, such as tank-fired kinetic energy projectiles, may be much smaller than the tube diameter. Such sub-caliber projectile bodies may be supported by (and guided along the tube with) a sabot, which discards upon exit from the tube. Once in flight, the fins are then a larger diameter than the diameter of the flight (projectile) body.
Some tube-launched projectiles may utilize folding fins or wrap-around fins to increase the aspect ratio of the fins (commonly used with tube-launched rockets and missiles). Such fins may be stowed within or near the body and may be deployed upon exit from the gun tube. The resultant super-caliber fins may add significantly to the projectile's stability. However, the cost, complexity and reduced reliability of these designs may be undesirable. These types of fins may increase the aerodynamic drag substantially, resulting in a loss of range. Most of these types of fins may have many moving parts. The moving parts must survive the harsh environment inside the gun tube (high pressures, temperatures and accelerations) and then deploy reliably upon exiting the tube.
To maximize the SSM, stabilizing surfaces may be located as far rearward as possible. Many times, however, the overall projectile length may be constrained due to packaging, storage or handling considerations. Thus, the stabilizer may not be moved far enough rearward to provide adequate stability. When faced with this dilemma, the aerodynamic designer may resort to the use of folding fins to provide additional tail lift. While folding fins may provide the necessary SSM, such a solution may be undesirable for the reasons described above.
For a full-caliber projectile with a constrained length, a need exists for a static stabilizer that mitigates or eliminates the disadvantages associated with known static stabilizers, such as folded fins.